An essential role for connexin 43 (Cx43) gap junctions in cardiac development is indicated by the finding that knockout mice deficient in Cx43 die at birth from heart defects. However, the precise role(s) of Cx43 gap functions in heart development remains unresolved. We do not known the specific cells and tissues in which they are required, or the precise function served by the Cx43 gap functions in these cells and tissues. Based on our previous studies and a large body of work by other laboratories, we have formulated the following overarching hypothesis. Hypothesis: Cx43 gap junctions play an essential role in heart development by modulating cell signaling in the cardiac neural crest cells and the cardiomyocytes. In Neural crest cells, Cx43 gap junctions mediate signaling involved in regulating the migration and activity of neural crest cells, thereby modulating events in cardiac development indirectly. In contrast, in cardiomyocytes, Cx43 gap junctions may mediate cell signaling involved in regulating myocyte contractility and cardiac conduction, thereby affecting cardiac development and function directly. To evaluate this hypothesis, we have formulated studies involving the use of transgenic mouse models and the analysis of mouse chimeras. The transgenic mouse approach is advantageous in that it allows the manipulation of Cx43 function in a tissue specific manner and will allow us to examine the role of Cx43 gap junctions specifically in neural crest cells vs. the myocytes. In contrast, the chimera studies are advantageous, because they constitute an independent and unbiased method for examining the role of Cx43 gap functions in cardiac development. Together, these will help to elucidate the role of CVx43 gap junctions in cardiac development, both in terms of identifying the cells and tissues in which Cx43 is required, and also its mode of action in these cells and tissues. These studies also are likely to be of clinical significance, as the heart malformations seen with experimental perturbation of Cx43 function resemble a class of congenital cardiovascular disorder referred to as pulmonic atresia with intact ventricular septum. In the long term, these studies will provide insights into the involvement of Cx43 gap junctions in lethal cardiac arrhythmias as well.